Various viscous liquid dispensers have been developed for the precise placement of a liquid, for example, cold and hot adhesives, nonadhesive liquids, etc. Often, a liquid dispenser has a valve stem with a valve body on its distal end which is disposed on an upstream side of a valve seat and moved in an upstream direction to open the valve and in a downstream direction to close the valve.
For purposes of this document, the term “upstream” refers to a direction or location that is toward, or closer to, the source or liquid inlet; and “downstream” refers to a direction or location that is away, or further, from a source or liquid inlet of the dispenser. Further, conical refers to a right cone; and a right cone is defined as a three dimensional shape formed by straight lines passing through a vertex forming one end of the cone and intersecting a circle in a plane forming an opposite end of the cone. The cone may have any spacial orientation.
With viscous liquids, the liquid being dispensed may adhere to itself as well as to the surface it contacts. Thus, during a dispensing process, adhesive forces may cause the viscous liquid to adhere to metallic nozzle surfaces surrounding a dispensing orifice. Adhesion of the dispensed liquid to the nozzle surfaces and the liquid's cohesive forces may result in an elongation of the dispensed liquid that is commonly referred to as tailing or stringing. In some applications, for example, in dispensing dots of viscous liquid, it is known to provide a generally circular wall of pressurized air around the dispensed liquid dot and its stringy tail, thereby directing the stringy tail into the top of the dispensed dot as it is being deposited on a substrate surface. The pressurized air is directed in a generally conical shape around the path of the dispensed liquid, and the air converges at a point generally coincident with the expected location of the dot on the substrate surface. This cone of pressurized air may direct the stringy tail into the center of the deposited dot and thus, may prevent the stringy tail from falling onto areas of the substrate surface that are not intended to be coated.
While this known system may direct the excursion of the viscous tail of the dispensed liquid dot in a desired manner, it does not address the problem of viscous liquid that may adhere to, and accumulate on, external nozzle surfaces surrounding the nozzle orifice. Such an accumulation or collection of material on external nozzle surfaces may change or adversely affect the quality of subsequent liquid dispensing operations. Such accumulation may interfere with, and/or alter, the location of the dispensed liquid on a substrate, which may result in scrap production. Further, wiping such accumulations off the nozzle surfaces may interrupt an otherwise automatic liquid dispensing process and may create process inefficiencies.